1. Field of Invention
This invention relates to metallic orthopedic implants with load bearing surfaces coated with a thin, dense, low friction, highly wear-resistant coating of zirconium oxide, nitride, carbide or carbonitride. This coating is especially useful on the portions of these prostheses which bear against surfaces which are subject to high rates of wear. An example is the femoral head of a hip-stem prosthesis which engages a counter-bearing surface in an acetabular cup which is often made of a softer material such as ultra-high molecular weight polyethylene.
The invention also relates to zirconium oxide coatings on the non-load bearing surfaces of an orthopedic implant where the zirconium oxide provides a barrier between the metallic prosthesis and body tissue thereby preventing the release of metal ions and corrosion of the implant. Additionally, this oxidation process and the associated increase in surface oxygen content and hardness increases the strength of the metal substrate and improves the fatigue properties of the implant.
2. Background
Orthopedic implant materials must combine high strength, corrosion resistance and tissue compatibility. The longevity of the implant is of prime importance especially if the recipient is relatively young because it is desirable that the implant should function for the complete lifetime of a patient. Because certain metal alloys have the required mechanical strength and biocompatibility, they are ideal candidates for the fabrication of prostheses. 316L stainless steel, chrome-cobalt-molybdenum alloys and more recently titanium alloys have proven to be the most suitable materials for the fabrication of load-bearing prostheses.
One of the variables affecting the longevity of load-bearing implants such as hip-joint implants is the rate of wear of the articulating surfaces and long-term effects of metal ion release. A typical hip-joint prosthesis includes a stem, a femoral head and an acetabular cup against which the femoral head articulates. Wear of either or both of the articulating surfaces results in an increasing level of wear particulates and "play" between the femoral head and the cup against which it articulates. Wear debris can contribute to adverse tissue reaction leading to bone resorption, and ultimately the joint must be replaced.
The rate of wear of the acetabular cup and the femoral head surfaces is dependent upon a number of factors which include the relative hardness and surface finish of the materials which constitute the femoral head and the acetabular cup, the frictional coefficient between the materials of the cup and head, the load applied and the stresses generated at the articulating surfaces. The most common material combinations currently used in the fabrication of hip-joint implants include femoral heads of cobalt or titanium alloys articulating against acetabular cups lined with organic polymers or composites of such polymers including, for instance, ultra-high molecular weight polyethylene and femoral heads of polished alumina in combination with acetabular cups lined with an organic polymer or composite or made of polished alumina.
Of the factors which influence the rate of wear of conventional hip-joint implants, the most significant are patient weight and activity level Additionally, heat which is generated by friction in the normal use of the implant as, for instance, in walking has been shown to cause accelerated creep and wear of the polyethylene cup. Furthermore, there is a correlation between the frictional moment which transfers torque loading to the cup and the frictional coefficient between the femoral head and the surface of the acetabular cup against which the head articulates. Cup torque has been associated with cup loosening. Thus, in general, the higher the coefficient of friction for a given load, the higher the level of torque generated. Ceramic bearing surfaces have been shown to produce significantly lower levels of frictional torque.
It is also noteworthy that two of the three commonly used hip-joint systems as indicated above include a metallic femoral head articulating against a UHMWPE liner inside the acetabular cup. UHMWPE, being a polymeric material, is more susceptible to creep when heated than the commonly used metal alloys or ceramics and is consequently more susceptible to wear than the alloys or ceramics.
It has also been found that metal prostheses are not completely inert in the body. Body fluids act upon the metals causing them to slowly corrode by an ionization process thereby releasing metal ions into the body. Metal ion release from the prosthesis is also related to the articulation and rate of wear of load bearing surfaces because, as may be expected, when a metallic femoral head, for instance, is articulated against UHMWPE, the passive oxide film which forms on the femoral head is constantly removed. The repassivation process constantly releases metal ions during this process. Furthermore, the presence of third-body wear (cement or bone debris) accelerates this process and micro fretted metal particles can increase friction. Consequently, the UHMWPE liner inside the acetabular cup, against which the femoral head articulates, is subjected to accelerated levels of creep, wear, and torque.
U.S. Pat. No. 4,145,764 to Suzuki et al recognized that while metal prostheses have excellent mechanical strength they tend to corrode in the body by ionization. Suzuki et al also recognized the affinity between ceramics and bone tissue, but noted that ceramic prostheses are weak on impact resistance. Suzuki et al therefore proposed a metal prosthesis plasma sprayed with a bonding agent which is in turn covered with a porous ceramic coating which would allow the ingrowth of bone spicules into the pores. This combination, it was said, would provide both the mechanical strength of metals and the biocompatibility of ceramics.
The Suzuki patent did not address the issue of friction or wear of orthopedic implant bearing surfaces but confined itself to the single issue of the biocompatibility of metal prostheses. Furthermore, Suzuki et al did not address the issue of dimensional changes that occur when applying a coating or the effect of these dimensional changes in the tightness of fit between the surfaces of an articulating joint prosthesis.
In addition, the application of ceramic coatings to metal substrates often results in non-uniform, poorly-bonded coatings which tend to crack due to the differences in thermal expansion between the ceramic and the underlying metal substrate. Furthermore, such coatings are relatively thick (50-300 microns) and since the bond between the metal and the ceramic coating is often weak there is always the risk of galling or separation of the ceramic coating.
U.S. Pat. No. 3,677,795 to Bokros is directed to the application of a carbide coating over a metallic prosthetic device. This method of forming the carbide coating requires that the prosthesis be heated to temperatures of at least about 1350.degree. C. in a reaction chamber through which a hydrocarbon gas such as propane or butane flows. The method is said to produce a prosthetic device which has "excellent compatibility with body tissue and is non-thrombogenic." Bokros does not address the issues of friction, heating, creep and wear of orthopedic implant bearing surfaces, or changes induced in the mechanical properties of the underlying metal due to this high-temperature treatment.
There exists a need for a metal alloy-based orthopedic implant having low friction, highly wear resistant load bearing surfaces which may be implanted for the lifetime of the recipient. There also exists a need for a metal alloy-based orthopedic implant that is not prone to corrosion by the action of body fluids so that it is biocompatible and stable over the lifetime of the recipient.